[mana]

hi all
I want to prepare a solution of NaOH in my book, it has mentioned for a 0.1 M solution of 250 ml sodium hydroxide solution, I should use 1.11 gr NaOH, the calculation is as below, I can understand all of the calculation except the final part, pls take a look and help me

? gr NaOH=250ml (solution)* (0.1 mol NaOH/1000ml solution )* (40gr NaOH/ 1mol NaOH)* (100 g solution/90gr NaOH)=1.11 gr NaOH

I can't understand why do I need to multiple at  (100 g solution/90gr NaOH)?

[AWK]

Seems to be 90 % NaOH, not a pure one.

[mana]

Seems to be 90 % NaOH, not a pure one.

THANKS